saltpeter, fifteen per cent of charcoal, and ten per cent. of sulphur.
The most important of the high explosives are formed by the action of
nitric acid upon organic substances or other hydrocarbons, the
compound radical NO2 being substituted for a portion of the
hydrogen in the substance. The bodies thus formed are in a condition
of unstable equilibrium; but if well made from good material, they
become stable in their instability, very much like Prince Rupert's
drops, those little glass pellets which endure almost any amount of
rough usage; but once cracked, fly into infinitesimal fragments.

The power exerted by these nitro-substitution products is due to the
fact that they detonate, i.e., they are instantaneously converted into
colorless gas at a very high temperature, and in addition they have
almost no solid residue. Nitro-glycerine actually leaves none at all,
while gunpowder leaves sixty-eight per cent. The first departure in
gunpowder from the old-time constituents of black powder just
mentioned was for the purpose of obtaining less pressure and slower
combustion than could be produced by mere granulating or caking. This
was accomplished by using underburned charcoal, together with sugar
and about one and one-half per cent. of water. This is the brown
powder most generally used at present and with satisfactory results;
but the abstraction of its moisture increases its rapidity of
combustion to a dangerous degree, besides which the underburned
charcoal is itself unstable.

The next change demanded is smokelessness, and to accomplish it
recourse is had to the high explosive field, mechanically mixing
various substances with them to reduce and regulate their rapidity of
action. Just now some form of gun-cotton is most in use mixed with
nitrate of ammonia, camphor and other articles. The tendency of these